Disk drives are a commonly used data storage device which utilise magnetic medium. It is required that the disk drive spindles have a low magnitude of random vibration in both the axial and radial directions. In addition, the basic requirements for practical magnetic recording devices are: high stiffness, especially in the radial direction, high shock resistance, and being capable of operating normally despite the orientation of the spindle axis. As consumer demands for the computer products, in particular personal computers, continuously push the advances of the magnetic recording technology toward miniaturisation, high storage capacity, and fast data transfer rate, the shortcomings of the conventional ball bearing supported spindle assembly becomes more and more acute. Major problems with the usage of a ball bearing spindle assembly are its high level of non-repeatable-runout (NRR), and wear rate. Because of the non-uniformity and geometric imperfectness of bearing balls, inner and outer races, unpredictable runout can occur during operation. This represents the main constrains for the data storage track width which has to accommodate the magnitude of the irregular vibration in the radial and the axial directions. As a result, the maximum achievable track density is limited primarily by the level of NRR. In practice, pre-loading of the ball bearing system of the spindle is used to reduce the NRR. However, excessive pre-loading force causes further increase in the wear rate and frictional losses, whilst any further miniaturisation of the disk drives necessitates lower power losses since heat dissipation becomes more difficult.
Another shortcoming of ball bearing spindle motors is that a seal mechanism is required to prevent any wear debris, dust, foreign bodies and evaporated substances from exhaling out of the bearings and contaminating the magnetic data storage media. The performance of this seal mechanism tends to degrade with increased wear and it is therefore not suitable for high speed applications. A further limitation arises with prior art ball bearing systems because scalable reduction of the size of the ball bearing structures is subject to unavoidable limitations and cannot always fit in the progressively miniaturised disk drives.
Fluid film bearings (FFB) appear to be a promising alternative to satisfy the demands for high precision spindles suitable for high capacity magnetic recording systems. In a self-acting FFB system, the bearing surfaces are kept separated by a lubricant film. That is, there is no metal to metal rubbing during operation, and therefore a comparatively low wear rate. The most outstanding feature of this bearing structure, however, is that is provides extremely low NRR spinning, as compared with the ball bearing spindle. The spindle motor structure described in U.S. Pat. No. 4,200,344 to Binns is one of the earliest inventions which makes use of the FFB technology. Like many others, for example, that described in U.S. Pat. No. 4,656,545 to Kakuta, it operates unidirectional, which limits the commercial applications as modern disk drives are required to operate regardless the shaft orientation.
One particular problem with the application of the cylindrical FFB journal bearing system is that it may lose its stability during operation. This is because the radial stiffness of the FFB journal bearing is low when the radial load is small, for example, when the spindle axis is vertically oriented. A particular consequence is instability due to sub-rotating-frequency whirl. As an economic and effective approach to achieve an enhanced bearing stability, tapered or conical bearings can be used. U.S. Pat. No. 4,734,606 to Hajec discloses electric motor constructions with tapered FFB lubricated by ferrofluid. Tapered air-lubricated FFB spindles for magnetic recording devices are described in the U.S. Pat. No. 5,283,491 to Jabbar. leakage of lubricant into the space receiving the data storage disks causes contamination of the recording media and thus malfunction of the disk drives. Leakage of the lubricant can also cause degradation of the bearing performance, resulting in failure of the disk read and write processes of disk drive systems. The seal may also cause excess frictional loss, for example, if a contact seal is used. The seal must additionally be designed to withstand a high level of shock. Other design options using sophisticated sealing mechanisms may only be realised at a high manufacturing cost. An example of such a hard drive spindle is disclosed in U.S. Pat. No. 5,246,294 to Pan.
Gas (including air) lubricated bearing spindles which avoid the necessity to seal the lubricant are attractive. In another spindle unit described in the U.S. Pat. No. 5,127,744 to White et al., a part of the stationary shaft is fitted with a rotationary ceramic sleeve to form the journal bearing, whilst two-disc-shaped ceramic thrust bearings are disposed at both ends of the bearing sleeve. A bearing structure of this kind, that is, two thrust bearings disposed at two ends of a journal bearing, occupies a significant portion of the space in the spindle hub. It may be suitable for use in the "underslung" topology for the spindle motor, in which the motor components are placed below the spindle hub.
In practice, gas lubricated bearing spindles suffer from the major weakness of difficulty in achieving sufficient stiffness. In this respect, the stiffness of the journal bearing is of far more concern since the required radial stiffness is not only critical but also, taking the magnetic disk drives by way of an example, must be higher than the required axial stiffness in many cases. Measures to enhance the rigidity of the air bearing system, for example, to reduce the bearing clearance, will inevitably increase the manufacturing cost. Furthermore, practical disk drives must be designed to withstand a high level of shock. When the rotating mass is large, for instance for high capacity disk drives which have a large number of disks, the stiffness constraint becomes more stringent. As a result, the utilisation of the self-acting FFB system lubricated fully by gas has been limited to cases where the passive load is relatively light, for example, the spindle of the polygon mirror scanners.